This invention relates to ink jet or liquid drop printing systems. More specifically, this invention relates to improved method and apparatus for optically detecting the accuracy of drop placement on a target and for other drop sensing or detecting goals.
Peter Crean and Paul Spencer disclose in their U.S. Pat. No. 4,255,754 Ser. No. 21,420, filed Mar. 19, 1979 a drop sensor using optical fiber pairs to sense drops in flight toward a target. The sensor typically includes an input fiber that is coupled to a light source at a remote end. The free or light emitting end is generally spaced from and centered to the free ends or light collecting ends of pair of output fibers (called A and B fibers herein). The remote ends of the output fibers are coupled to separate photodetectors. The photodetectors are in turn coupled to a differential amplifier.
The light collecting ends of the A and B fibers are oriented along the x axis of an x, y and z axis orthogonal coordinate system. The drops travel along a flight path generally in the z direction. The target to be printed lies generally in the x-y plane and is moving generally in the y direction. The shadow of a drop flying between the input fiber and the A and B output fibers generates an electrical positional signal from the differential amplifier. If the drop shadow falls evenly (or some other reference criteria) onto the collecting faces of the A and B fibers the drop is aligned to the bisector between the two fibers. This, of course, in the orientation described, corresponds to a precise point, e.g. x.sub.o, along the x axis of the coordinate system. If the drop shadow is unbalanced on either the A or B fiber, a plus or minus position signal is generated by the differential amplifier. The magnitude of the position signal indicates a precise position left or right on the x axis of x.sub.o. Typically, the position signal is coupled back to a drop charging and deflection mechanism to servo the drops directly over the x.sub.o position.
The Crean et al patent describes the use of the optical fiber sensors to calibrate drop charging levels for a plurality of drop streams. This process enables drops from each stream to be precisely positioned to multiple pixel positions within a segment of a scan or print line that extends across the target. Consequently, the print line segments of the adjacent drop streams are said to be "stitched."
Other multiple drop stream liquid drop systems also have need for detecting the position of a drop relative to a reference point or line. An example is the binary drop printing system of the type disclosed in U.S. Pat. No. 3,373,437 to Sweet and Cumming. Systems of this type continuously generate a plurality of drops in flight toward a target. The drops from each stream are able to reach only a single pixel within a scan line on the target. The drops not intended for the target are collected in gutter. The binary aspect is therefore the alternate selection of each drop in a stream for flight to the intended pixel on a target or collection in a gutter.
Drop position sensing in the above and other types of liquid drop systems is important for checking drop velocity and charge phasing as well as drop location. Generally, it is more useful to sense charged rather than uncharged drops because their trajectories are usually correctable by altering the charge on a drop prior to its flight through an electrostatic deflection field. However, the liquid pressure in the drop generator can be varied to affect the trajectory of the uncharged drops.
The Crean et al optical fiber sensors involve a large number of fibers with the A and B fibers being separated into groups for termination at first and second photodetectors. All (or a large number of) the A fibers share the same first photodetectors and all (or a large number) of the B fibers share the same second photodetectors. Since the light collecting ends of the plural sets of A and B fibers lie in the same x-y plane, the A fibers must cross over the B fibers for the two types to be grouped together. That is, the fibers are organized into groups that intersect each other thereby necessitating that the A fibers be crossed over the B fibers or vice versa. This can be done with individual fibers but makes for difficult assembly of large numbers of sensors. Heretofore, the cross-over problem has made integrated waveguide structures impractical for sensing arrays.
SUMMARY
Accordingly, it is a main object of this invention to overcome limitations of assembly of multiple fiber drop sensors employed in liquid drop printing systems.
Specifically, it is an object of the invention to organize the A and B optical fibers of the Crean et al type of sensor array into non-intersecting groups of A and B fibers to improve the manufacturability of the array while preserving the sensing function of the individual fibers.
Yet another object here is to employ laminar optical fiber structures in sensor arrays of the type disclosed in the Crean et al patent.
The above and other objects of this invention are realized by offsetting the light collecting ends of each A and B fiber into parallel and separate planes, at least at the sensing zone. In one embodiment, the A fibers of multiple fiber pairs are formed on a support surface of a single substrate member. The A fibers are coated over with an appropriate separation material creating a second support surface. The B fibers are formed on this second support surface. In another embodiment, the A, B and input fibers are formed on separate substrates. The A and B fibers are then oriented at the sensing zone as explained.
Detection circuits coupled to the remote ends of the A and B fibers store the signals associated with a drop shadow striking the A and B fibers. The storage is provided because the two signals are generated at different times. The delay is due to the separation between the A and B fibers along the z axis, i.e. the direction of flight.